Mechanical evaluation of pedicle screw fixation of the lumbar spine

Pedicle screw fixation of the lumbar spine has been reported to increase fusion rates A biomechanical evaluation of four different pedicle screw implant systems, (AO. Rogozinski, TSRH and Wiltse). was performed to compare intrinsic device stiffness under conditions of flexion-compression and forty-five degree off-axis flexion-compression The effect on stiffness of the loosening of device members was also studied. Testing was done in load control using an electrohvdrualic testing machine. UHMWPe blocks are used to simulate the vertebra. Assuming that stiffness is directly proportional to the probability of obtaining fusion. this study allows the ranking of the systems tested in their normal loading stiffnesses and their abilities to maintain stiffness with off axis loading and unintentional loosening of components. This study indicates a ranking of the four systems tested as TSRH being the most stiff followed by AO and Wiltse. Clearly, the worst system tested, from consideration of initial stiffness, off-axial load and loosening is the Rogozinski construct